Systematics of Acacia

Project summary

Our aim is to discover the phylogenetic relationships within the Australian Acacia clade in order to produce a predictive, phylogeny-based classification.

Acacia in the strict sense (formerly Acacia subgenus Phyllodineae) is the second largest genus of legumes (after Astragalus) and the largest genus in subfamily Mimosoideae (syn. Mimosaceae). This is even after the segregation of Acacia in the broad sense into five genera based on recent phylogenetic research.

Phylogenetic analyses of molecular and morphological data show that Acacia in the strict sense is a single evolutionary lineage (monophyletic), almost completely confined to Australia. Few plant clades dominate a continent as extensively as Acacia. Recent estimates of species numbers range from 986 species to about 1,045 species, when undescribed taxa are included. Acacia is widespread within Australia and forms a dominant component of many vegetation classes, particularly in Australia's large arid and semi-arid zone.

Acacia is commonly divided into seven sections, characterised by combinations of easily recognised macro-morphological characters. Results of molecular phylogenetic studies have demonstrated that the traditional sections are largely non-monophyletic and, as such, there is an acute need for an infrageneric classification of Acacia based on phylogenetic relationships.

There are at least four main clades in Acacia, the following were identified based on nuclear ribosomal DNA:

A clade, resolved as sister to all other taxa in Acacia, including species related to A. victoriae and A. pyrifolia

The Pulchelloidea clade, which includes members of sections Pulchellae, Alatae, Phyllodineae and Lycopodiifoliae. Gillian Brown is currently analysing a large sample of taxa, which are putatively placed in this clade, as part of an Australian Research Council (ARC) funded Linkage grant. Sampling has been undertaken in consultation with morphological information provided by Bruce Maslin.

A third clade comprises taxa in the A. murrayana species group. These taxa occur predominantly in semi-arid and arid regions.

A fourth clade consists of a diverse assemblage of phyllodinous Acaciasepcies. We have dubbed this the 'p.u.b. clade', to include all those taxa with 'plurinerved' phyllodes (in sections Juliflorae and Plurinerves), some uninerved phyllodinous taxa (sect. Phyllodineae) and eastern Australian bipinnate taxa (sect. Botrycephalae).

We have analysed the phylogeny of the phyllodinous Acacia species that occur outside Australia, discovering that all these taxa are nested within Acacia, although not as a single lineage. Theseseparate clades of Acacia found outside Australia, with close relationships to Australian species, indicate different timings of range expansion for Acacia. An intriguing prospect is the human-mediated transport of species through a connection between taxa in the Indian Ocean (A. heterophylla) and Hawaii (A. koa and relatives).

Our research group is also currently carrying out studies on the close relatives of Acacia in the mimosoid legume tribe Ingeae. We have discovered that the sister group to Acacia is Paraserianthes lophantha, a member of the tribe Ingeae and a widespread weed, with a disjunct natural distribution in Western Australia and Indonesia.

Reid, J.C. and Murphy, D.J. (2008). Some case studies of Acacia as weeds and implications for herbaria. Muelleria26, 57–66.

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